A spatial light modulator imposes some form of spatially varying modulation on a light beam in order to project two or three-dimensional images. These modulations concern for example the phase and/or the amplitude of the light beam.
A spatial light modulator usually comprises an array of controllable elements adapted for generating a modulated light beam by diffracting an incident light beam. This array of controllable elements forms a pixelated structure.
However, because of the pixelated structure of the spatial light modulator, a small fraction of the incident light beam, usually a couple of percents, finds itself in the inter-pixel space and therefore is never diffracted by the controllable elements. This undiffracted part of the modulated light beam is called “zero-order” part. The “zero-order” part may also originate from other technical limitations such as an imperfect anti-reflection coating of the spatial light modulator and some inter-pixel cross-talk limiting the control over the controllable elements. This undiffracted part of the modulated light beam cannot be suppressed or moved away by the spatial light modulator itself.
Now referring to FIG. 1, an optical system 1 for holographic photo-excitation comprising a spatial light modulator 2 is illustrated. The optical system 1 aims to project an image on a biological sample arranged on a support 3.
The optical system 1 comprises a laser light emitting source 4 generating an incident light beam 5. The spatial light modulator 2 is arranged on the optical path of the incident light beam 5 for diffracting said incident light beam 5 and generating a modulated light beam 6. As explained above, the modulated light beam 6 comprises a modulated part carrying an image and an unmodulated “zero-order” part. The optical system 1 also comprises an objective-lens assembly 7 including a telescope and a microscope objective for projecting the image on the biological sample and exciting said biological sample, as well as an optical arrangement 8 made of optics and detector for observing the excited biological sample on a display.
As shown in FIG. 2, wherein the image to be projected on the sample has been chosen to be the Eiffel tower, the unmodulated “zero-order” part results in a tightly focused spot illuminating the sample that may induce unwanted excitation of the sample and therefore artifacts in the obtained result.
Several researches have been carried out in order to avoid that the unmodulated “zero-order” part illuminates or excites the sample. Such researches are summarized in the document Zhang et al., Elimination of a zero-order beam introduced by a pixelated spatial light modulator for holographic projection, Applied Optics, vol. 48, No. 30, 5834 (20 Oct. 2009).
A first technique according to this document is to block the unmodulated “zero-order” light beam in an intermediate plane, so that the modulated part carrying the image is the only one illuminating the biological sample. Blocking the zero-order in an intermediate image is very efficient but introduces a blind-zone at the center of the excitation field. Moreover, it requires a precise alignment of a supplementary optical element which is the beam block. In addition, diffraction efficiency is degraded as the pattern of interest must be moved away (transversely or axially) from the central blind-zone.
A second technique according to this document is to shift away the unmodulated “zero-order” part from the region of interest of the biological sample, so that the region of interest is only illuminated by the modulated part carrying the image. Such a technique is for example illustrated in FIG. 3 wherein the image to be projected on the sample has been chosen to be a disk. As shown in FIG. 3, the unmodulated “zero-order” part has been shifted away from the region of interest of the sample, i.e. the region on which the disk is projected. However, this technique may still induce unwanted excitation of the sample by the unmodulated “zero-order” part and therefore artifacts in the obtained result. This technique may also decrease the diffraction efficiency of the spatial light modulator due to the required modification of the image signal component of the control signal.
The present invention aims to provide a technique for suppressing the artifact due to the unmodulated «zero-order» part of the modulated light beam without inducing a blind zone at the center of the replay volume nor inducing unwanted excitation of the sample.